i BYZANTINE ARCHITECTURE 5 gallery is placed above it, both narthex and gallery are covered by barrel vaults. In the domed cross church (Kreutzkuppelkirche) the central dome rests on barrel vaults which extend to the outer walls of the building and form the arms of the cross, the eastern arm forming the beraa. The lighting of the church is by windows in the gable walls which terminate the north, south, and west cross arms. The prothesis and diaconicon open off the side arms, and two small chambers in the western angles of the cross bring the plan externally to the 'usual rectangular form. The domed cross church may have galleries, as in S. Theodosia (p. 170)3 or may be without them, as in SS. Peter and Mark (p. 193). Where galleries are present they are placed in the cross arms and are supported by arcades at the ground level. The vaults beneath the galleries are cross-groined. The domed cross church is a centrally planned church, in contrast to the domed basilica, which is oblong, and therefore we should expect that where galleries are used they will be formed in all three arms of the cross, as is the case in S. Theodosia. There are a number of churches which vary from these types, but which can generally be placed in one class or the other by the consideration of two main characteristics : if the dome arches extend to the outer walls the building is a domed cross church ; if the galleries are screened off from the central area by arcades the building is a domed basilica. The church at Dere Aghsy,1 for instance, if we had only the plan to guide us, would appear to be a typical domed basilica (Fig. 2), but on examining the section we find that the north and south dome arches extend over the galleries to the outer walls and form cross arms (Fig. 3). The building is, in fact, a domed cross church with no gallery in the western arm. Above the narthex at the west end, and separated from the western cross arm, is a gallery of the type usual in the domed basilica, so that Der& Aghsy may be regarded as a domed cross church with features derived from the domed basilica. S. Sophia at Constantinople, 1 Oskar Wulf, Die Koimesiskirche in Nikaea, p. 71.